Running a test on a large CPU count box with xfs, I hit a live lock
with the following backtraces on several CPUs:
Call Trace:
[<ffffffff812c34f8>] __const_udelay+0x28/0x30
[<ffffffffa033ab9a>] xfs_icsb_lock_cntr+0x2a/0x40 [xfs]
[<ffffffffa033c871>] xfs_icsb_modify_counters+0x71/0x280 [xfs]
[<ffffffffa03413e1>] xfs_trans_reserve+0x171/0x210 [xfs]
[<ffffffffa0378cfd>] xfs_create+0x24d/0x6f0 [xfs]
[<ffffffff8124c8eb>] ? avc_has_perm_flags+0xfb/0x1e0
[<ffffffffa0336eeb>] xfs_vn_mknod+0xbb/0x1e0 [xfs]
[<ffffffffa0337043>] xfs_vn_create+0x13/0x20 [xfs]
[<ffffffff811b0edd>] vfs_create+0xcd/0x130
[<ffffffff811b21ef>] do_last+0xb8f/0x1240
[<ffffffff811b39b2>] path_openat+0xc2/0x490
Looking at the code I see it was stuck at:
STATIC void
xfs_icsb_lock_cntr(
xfs_icsb_cnts_t *icsbp)
{
while (test_and_set_bit(XFS_ICSB_FLAG_LOCK, &icsbp->icsb_flags)) {
ndelay(1000);
}
}
I'm not sure why it does the ndelay() and not just a cpu_relax(), but
that's besides the point. In xfs_icsb_modify_counters() the code is
fine. There's a preempt_disable() called when taking this bit spinlock
and a preempt_enable() after it is released. The issue is that not all
locations are protected by preempt_disable() when PREEMPT_RT is set.
Namely the places that grab all CPU cntr locks.
STATIC void
xfs_icsb_lock_all_counters(
xfs_mount_t *mp)
{
xfs_icsb_cnts_t *cntp;
int i;
for_each_online_cpu(i) {
cntp = (xfs_icsb_cnts_t *)per_cpu_ptr(mp->m_sb_cnts, i);
xfs_icsb_lock_cntr(cntp);
}
}
STATIC void
xfs_icsb_disable_counter()
{
[...]
xfs_icsb_lock_all_counters(mp);
[...]
xfs_icsb_unlock_all_counters(mp);
}
STATIC void
xfs_icsb_balance_counter_locked()
{
[...]
xfs_icsb_disable_counter();
[...]
}
STATIC void
xfs_icsb_balance_counter(
xfs_mount_t *mp,
xfs_sb_field_t fields,
int min_per_cpu)
{
spin_lock(&mp->m_sb_lock);
xfs_icsb_balance_counter_locked(mp, fields, min_per_cpu);
spin_unlock(&mp->m_sb_lock);
}
Now, when PREEMPT_RT is not enabled, that spin_lock() disables
preemption. But for PREEMPT_RT, it does not. Although with my test box I
was not able to produce a task state of all tasks, but I'm assuming that
some task called the xfs_icsb_lock_all_counters() and was preempted by
an RT task and could not finish, causing all callers of that lock to
block indefinitely.
Looking at all users of xfs_icsb_lock_all_counters(), they are leaf
functions and do not call anything that may block on PREEMPT_RT. I
believe the proper fix here is to simply disable preemption in
xfs_icsb_lock_all_counters() when PREEMPT_RT is enabled.
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
index 51435dbce9c4..dbaa1ce3f308 100644
--- a/fs/xfs/xfs_mount.c
+++ b/fs/xfs/xfs_mount.c
@@ -1660,6 +1660,12 @@ xfs_icsb_lock_all_counters(
xfs_icsb_cnts_t *cntp;
int i;
+ /*
+ * In PREEMPT_RT, preemption is not disabled here, and it
+ * must be to take the xfs_icsb_lock_cntr.
+ */
+ preempt_disable_rt();
+
for_each_online_cpu(i) {
cntp = (xfs_icsb_cnts_t *)per_cpu_ptr(mp->m_sb_cnts, i);
xfs_icsb_lock_cntr(cntp);
@@ -1677,6 +1683,8 @@ xfs_icsb_unlock_all_counters(
cntp = (xfs_icsb_cnts_t *)per_cpu_ptr(mp->m_sb_cnts, i);
xfs_icsb_unlock_cntr(cntp);
}
+
+ preempt_enable_rt();
}
STATIC void
--
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